A general optical receiver in optical communication is formed from a light-receiving element such as a photodiode or an avalanche photodiode, and a transimpedance amplifier configured to amplify a photocurrent generated by the light-receiving element. The light-receiving element has a function of converting incident light into a current. In the light-receiving element, a light absorption layer absorbs light, thereby generating pairs of electrons and holes as carriers. When the generated electrons and holes move, a current flows.
The upper limit of the photoelectric conversion efficiency of a photodiode is 100% as quantum efficiency. On the other hand, the avalanche photodiode is a light-receiving element having a function of making photoelectrons generated in the element hit a lattice by accelerating them under a high electric field and thus ionizing the photoelectrons, thereby amplifying the carriers. For this reason, the avalanche photodiode can output a plurality of carriers in correspondence with one photon and obtain a sensitivity higher than 100% as the quantum conversion efficiency. The avalanche photodiode is applied to a high-sensitivity optical receiver (see non-patent literature 1).
In the avalanche photodiode, however, the linearity between the input light intensity and the output electrical signal is poorer than in the photodiode. For this reason, if an optical receiver needs high linearity between the input light intensity and the output electrical signal because of a constraint on the distortion of an output waveform, like a digital coherent method recently under research and development, it is difficult to apply the avalanche photodiode.
This is because holes having a moving velocity lower than that of electrons are accumulated in the light absorption layer if the input light intensity is high. This phenomenon is called a space-charge effect. In the photodiode, the accumulated holes locally lower the field strength in an absorption layer. However, the linearity of the response is ensured until the field strength becomes almost 0. On the other hand, in the avalanche photodiode, since charges by the accumulated holes lower the field strength in an avalanche layer, the multiplication factor lowers in a case in which the input light intensity is high. For this reason, in the avalanche photodiode, the linearity of the electrical output strength with respect to the input light intensity is poorer than in the photodiode.
As a general means for solving the above-described problem of the avalanche photodiode, it is considered that a light absorption layer having a uni-traveling carrier photodiode (UTC-PD) structure is formed. In the UTC-PD structure, the light absorption layer is formed into p-type by doping. Hence, the linearity of the electrical output strength with respect to the input light intensity does not degrade in accordance with hole accumulation caused by a high input light intensity (see non-patent literature 2).
More specifically, a photodiode normally has a p-i-n photodiode (pin-PD) structure in which an undoped layer (i-layer) serving as a light absorption layer is sandwiched between a layer (p-layer) formed into p-type by doping and a layer (n-layer) formed into n-type by doping. In the pin-PD structure, carriers generated in the light absorption layer are accelerated by the electric field in the depleted undoped layer so that the carriers can move at a high speed. However, of the generated carriers, holes have a moving velocity lower than that of electrons. This is a factor to limit the operation speed.
An element having the UTC-PD structure eliminates the factor that limits the speed in the above-described pin-PD structure, thereby further speeding up the operation. In an undoped structure, the light absorption layer is undoped, and no carrier collecting layer is included. In the UTC-PD structure, however, the light absorption layer is of p-type, and a region (depletion layer) to be depleted at the time of an operation is formed as a carrier collecting layer made of a material different from the material of the light absorption layer, as shown in FIG. 11 (see non-patent literature 2). With this structure, minority carriers (electrons) generated in the light absorption layer can be diffused to the depleted carrier collecting layer. In addition, to prevent the electrons from diffusing back to the p-contact side, a p-type “diffusion block layer” is inserted between the p-contact and the light absorption layer.
Of the carriers generated in the p-type light absorption layer, the holes need a time corresponding to the dielectric relaxation time (on the 10−12 sec order) for response. Hence, the holes are not accumulated. That is, as the factor to decide the speed of the element, only the movement of electrons needs to be taken into consideration. The movement of electrons in the UTC-PD includes diffusion in the p-absorption layer and drift in the carrier collecting layer. When an overshoot effect is used in the carrier collecting layer, the drift time shortens, and an ultrahigh-speed operation at 100 GHz or more can be obtained.
However, the carrier transit time in a general undoped light absorption layer is inversely proportional to the absorption layer thickness, and the carrier transit time in the light absorption layer of the UTC-PD structure is inversely proportional to the square of the absorption layer thickness. That is, in the UTC-PD structure, if the absorption layer is made thin while making a great sacrifice of sensitivity, an ultrahigh-speed operation at 100 GHz or more can be obtained. However, in a band of several ten GHz, a higher sensitivity is obtained by an undoped light absorption layer in some cases. In the UTC-PD structure, the upper limit of the photoelectric conversion efficiency is 100% as quantum efficiency. Additionally, especially when the absorption layer is made thick to raise the sensitivity, the sensitivity largely degrades.
Note that in an avalanche photodiode using an undoped absorption layer, as shown in FIG. 12, an undoped light absorption layer, a p-field control layer, an InAlAs avalanche layer, an n-field control layer, an InP edge-field buffer layer, and an n-contact layer are provided on a p-type InP contact layer (see non-patent literature 3). In this structure, a sensitivity higher than 100% as the quantum conversion efficiency can be obtained by amplifying the carriers in the avalanche layer. However, the linearity is poor because the undoped absorption layer is used.